An uplink power control method that considers uplink interference levels received from neighbor sectors is defined in the IEEE 802.16 standard or the like.
The draft standard is based on a power adjusted value of an uplink feedback channel in order to set transmission power for uplink traffic burst transmission. In order to perform additional transmission power control in consideration of interference levels between neighbor sectors, each user terminal uses a distribution method that determines a power adjusted value. In this case, the feedback channel is used for the purpose of a channel quality report, a bandwidth request, a band selection, a report on a beam index for beamforming, and the like during an uplink.
The base station measures a ratio of received signal to interference and noise (SINR) of the feedback channel, and broadcasts a power adjusted value for satisfying a specifically required SINR through a downlink control channel to a user terminal. The user terminal adjusts transmission power for the feedback channel according to the power adjusted value that has been received through a downlink control channel.
In addition, transmission power of a traffic burst that is to be used during an uplink is set by adding a predetermined control-data power offset to the transmission power of the feedback channel.
Further, when the base station informs a user terminal of an uplink traffic burst allocation through the downlink control channel, the base station can set a correction power offset for additional power correction that is to be used during an uplink traffic burst transmission.
That is, the user terminal adds the predetermined control-data power offset and the correction power offset to the transmission power of the feedback channel, and sets power for traffic burst transmission.
In the draft standard, an additional power adjusting method in consideration of interference levels between neighbor sectors is included in the general uplink power control method.
First, the base station measures a level of interference that occurs at the time of an uplink transmission of a user terminal that belongs to a neighbor sector, broadcasts the measured interference level to the user terminal with a predetermined period, and transmits an interference value of another sector composed of 2 bits in all subcarriers belonging to one OFDM symbol such that user terminals belonging to another sector other than a corresponding sector, which the base station is responsible for, can receive an interference value of another sector.
The user terminal receives an interference value of another sector that is broadcasted from base stations of neighbor sectors other than a service sector, to which the user terminal belongs, in order to perform power control in consideration of interference levels of the neighbor sectors. The user terminal calculates a correction power offset on the basis of interference values of another sector for the neighbor sectors, and adds a correction power offset in consideration of a predetermined control-data power offset and interference of the neighbor sectors to transmission power of the feedback channel so as to set the uplink traffic burst transmission power.
If the base station informs the user terminal of a correction power offset set by the base station through a downlink control channel, the user terminal sets the correction power offset received from the base station instead of the correction power offset set by the user terminal as the uplink traffic burst transmission power.
The base station needs to know a transmission power offset used by each user terminal in order to appropriately set a modulation and coding scheme and a bandwidth used by each user terminal for the purpose of uplink traffic burst transmission. For this purpose, the user terminal transmits a power control report message that includes information on a power offset used by the user terminal and a maximum value of the number of subcarriers that can be applied by using transmission power of the user terminal to the base station.
The base station sets a modulation and coding scheme and a power offset necessary for uplink traffic burst transmission of the user terminal by using the power offset of the received power control report message and the maximum value of the number of subcarriers, and informs the user terminal thereof through the downlink control channel.
However, according to the power control method, the base station uses only one OFDM symbol in transmitting interference values of another sector composed of 2 bits such that when the base station broadcasts the received interference levels to the neighbor sector, the user terminals in another sector can also receive them. Therefore, the radio resource may be wasted.
Further, since an interference value of another sector is broadcasted from the base station of the neighbor sector, a range of the user terminal, which is capable of receiving the interference value of another sector, may be restricted.
Furthermore, since each user terminal needs to receive an interference value of another sector that is broadcasted from a neighbor sector, each user terminal needs to periodically receive not only a downlink signal of a sector corresponding to each user terminal but also a downlink signal transmitted from a neighbor sector.
Further, since each user terminal uses a power control report message in order to inform the base station of a correction power offset set by each user terminal, power control is not quickly reported to the base station. The reason is because the user terminal needs to perform various processes, such as an uplink band request, a band allocation through a control channel, or transmission of a power control report message, in order to transmit the power control report message in the uplink.
When the power control report that the base station receives from the user terminal does not exist or is not received for a long time, the base station performs uplink scheduling on the basis of information of a out-of-data power control report. For this reason, it is not possible to accurately perform uplink traffic burst transmission from the user terminal to the base station.
Furthermore, when the base station allocates the uplink traffic burst to the user terminal through the control channel, if it includes a correction power offset set by the base station, the user terminal performs power control by using the included correction power offset. Therefore, the user terminal cannot use a correction power offset that is selected in consideration of an interference situation and a channel situation of a neighbor sector.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.